Improvement in performance of liquid crystal display elements has become an essential issue with the recent development in information-oriented society. For higher processing speed and performance, liquid crystal compositions must contain a component having large refractive index anisotropy.
Tolan compounds are known as liquid crystal having relatively large refractive index anisotropy (Mol. Cryst. Liq. Cryst., Vol. 23, p233 (1973)). However, the refractive index anisotropy of this compound is about 0.2, which is not yet large enough.
There have also been developed compounds represented by the following formulae and disclosed in JP-2-83340-A and JP-9-216841-A:
(wherein “Alkyl” stands for an alkyl group)
(wherein R stands for an alkyl group; Y stands for R, a fluorine, chlorine, bromine, or iodine atom, or a cyano group; H1 to H12 each stands for a hydrogen, fluorine, or chlorine atom, provided that at least one of H1 to H12 stands for a fluorine or chlorine atom).
The refractive index anisotropy of these compounds is larger than that of the tolan compounds, but is yet as small as about 0.4. In the second compound, when the hydrogen atoms are substituted with halogen atoms such as fluorine atoms for improved compatibility, the refractive index anisotropy becomes still smaller, e.g. about 0.3. Under such circumstances, development of liquid crystalline compounds with larger refractive index anisotropy is demanded.
However, if the improvement in refractive index anisotropy is sought by extending the conjugated pi-electron systems in such compounds, peaks of the absorption spectrum of the compounds in the ultraviolet and visible regions shift to the longer wavelength side, sometimes resulting in undesirable coloring of the compound.
There have been discussed possible application of liquid crystalline materials not only to a switching element of displays for switching the display modes such as TN or STN mode, but also to retarders, polarizers, polarizing prisms, beam splitters, reflectors, holographic elements, color separators, or various optical filters, which make use of the optical anisotropy of the materials such as alignment and refractive index. Improvement in performance of display elements has also become an essential issue with the recent development of the information-oriented society.
As techniques for production of optically anisotropic products from such liquid crystalline materials, there are known, for example, methods of photopolymerizing a liquid crystalline compound having a polymerizable functional group, or a polymerizable liquid crystal composition containing such a compound, by irradiating the compound or the composition in a liquid crystal state with ultraviolet or visible irradiation. These methods intend, in other words, to produce polymers wherein the liquid crystal molecules aligned in the liquid crystal state are semipermanently fixed for achieving stable optical functions.
Recently known liquid crystalline compounds having a polymerizable functional group are disclosed in JP-A-11-116534 and JP-A-11-80090, the former proposing mainly a compound having a phenylbenzoate core, and the latter a compound having a core including phenylbenzoate, cyclohexylphenyl, and tolan. Neither of the compounds, however, has a core exhibiting particularly large refractive index anisotropy (Δn).
It is known that a substituent may be introduced into a dibenzothiophene compound through a process disclosed in J. Am. Chem. Soc., 1948, 70, 1748, or in Heterocyclic Chem., 1985, 22, 215. However, by these processes, it is hard to introduce two highly reactive substituents selectively into the 3- and 7-positions of a dibenzothiophene compound.